In U.S. Pat. No. 4,793,161 (the disclosure of which is hereby incorporated by reference herein), an extremely effective apparatus and method for the backflushing of screens in paper pulp treatment vessels has been provided. The system illustrated and described in that application supplies backflushing liquid from interior of the vessel itself rather than from the exterior of the vessel, resulting in numerous advantages, including eliminating surge.
While the techniques illustrated in said U.S. Pat. No. 4,793,161 have been very successful when applied to conventional single stage diffusers or related pulp treating apparatus, prior to the invention they were not successfully applied to conventional double diffusers, such as illustrated in U.S. Pat. No. 4,172,037. In conventional double stage diffusers, supporting arms/liquid conduits are located between two sets of screens, with pulp treatment liquid introducing nozzles located underneath the diffuser units and rotated by an extension of the top scraper shaft with supporting bearing. However it is even more desirable in double diffusers, than single diffusers, to utilize the internal backflushing means of said U.S. Pat. No. 4,793,161 since the surge in the double diffusers is essentially twice that in a single diffuser.
According to the present invention, it is possible to utilize the advantageous internal backflushing means associated with said U.S. Pat. No. 4,793,161 in a double diffuser by providing a particular construction of the double diffuser screens, and by providing a particular construction of the backflushing units.
According to one aspect of the present invention, the screens comprise a first set of screens supported by a plurality of supporting arms/liquid conduits, and a second set of screens, extending above the first set and connected thereto. First extraction means are provided for extracting liquid through the first set of screens, and second extraction means independent of the first means are provided for extracting liquid through the second set of screens. First and second screen backflushing means are provided for respectively flushing each of the first and second sets of screens. Each screen backflushing means comprises--in the preferred embodiment--a piston and cylinder arrangement. Preferably, the first and second cylinders are concentric with each other, both having open ends communicating with the liquid conduit/supporting arms, and the first and second pistons are mounted on a common piston rod. A linear actuator--such as one mounted on top of the vessel or the like--is provided for effecting reciprocation between the pistons and cylinders, to provide the backflushing action. One face of the first piston is mounted in operative communication with the pulp, while its second face--and both faces of the second piston--are in operative communication with liquid within the liquid conduit/supporting arms.
Preferably treatment liquid introducing nozzles are provided for introducing treatment liquid into the vessel. The treatment liquid introducing structures preferably comprise first and second sets of nozzles mounted to a common rotating element at the top of the vessel. The first nozzles extend down into operative association with the first set of screens, and the second nozzles extend into operative association with the second set of screens. The pulp treating apparatus may be a diffusion washer, a displacement bleacher, or the like.
According to another aspect of the present invention a pulp treating apparatus may be provided which comprises: A generally upright vessel defining an interior volume containing pulp. Liquid conduit means mounted in the vessel. Liquid pumping means located within the vessel and comprising first and second chamber defining elements having first and second ends, respectively, the first end of the first chamber in open communication with pulp in the interior volume of the vessel, and the second end of the first chamber and the first and second ends of the second chamber in open communication with liquid in the liquid conduit. First and second liquid engaging elements mounted within the first and second chamber defining elements, respectively, for relative reciprocal movement; and means for effecting relative movement between the chamber defining elements and the liquid engaging elements to cause the liquid engaging elements to force liquid out of the chambers and into the liquid conduits in one direction of relative movement, and to take the liquid into the chambers from the liquid conduit means in another direction of relative movement.
According to another aspect of the present invention, a pulp treating apparatus is provided which comprises: A generally upright vessel defining an interior volume containing pulp. Liquid conduit means mounted in the vessel. A first cylinder open at its first end to the pulp in the vessel, and at its second end to the liquid in the conduit means. A second cylinder open at both the first and second ends thereof to the liquid in the conduit means. A first piston, with piston rod, mounted in the first cylinder; and a second piston, with piston rod, mounted in the second cylinder. And, means for effecting relative reciprocation between the pistons and cylinders.
It is the primary object of the present invention to provide for the effective backflushing of a double diffuser type of construction of pulp treatment vessel (e.g. diffusion washer), without surge or other adverse effects such as are common in prior art double diffusers. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.